It's being played as a near-colllision, which it wasn't; just maintaining recommended interval to stay out of the C17's tip turbulence.
Turbulence from the wingtip vortices behind a heavy lifter is a very real concern and much greater behind a plane taking off than one landing. Many times I've seen planes stacked for landing at Newark that were no way in Hell more than three miles apart, tops, but of course they were landing one right after the other, not trying to land behind a plane taking off. Due to cross-winds, the turbulent air generally does not stay lined up on the flight path of the plane that created it, and it's invisible, which results in an unpredictable asymmetric downdraft or updraft effect on any other plane that gets too close behind it, which is potentially disastrous in final approach.
It appears from what I've heard about it that the error occurred by a civilian ATC at the regional level giving her plane the go-ahead, while the ATC in the tower at Andrews was not ready to accept her plane on the hand-off due to the C17's departure.